#if NETSTANDARD
namespace System;

/// <summary>Represent a range has start and end indexes.</summary>
/// <remarks>
/// Range is used by the C# compiler to support the range syntax.
/// <code>
/// int[] someArray = new int[5] { 1, 2, 3, 4, 5 };
/// int[] subArray1 = someArray[0..2]; // { 1, 2 }
/// int[] subArray2 = someArray[1..^0]; // { 2, 3, 4, 5 }
/// </code>
/// </remarks>
/// <remarks>Construct a Range object using the start and end indexes.</remarks>
/// <param name="start">Represent the inclusive start index of the range.</param>
/// <param name="end">Represent the exclusive end index of the range.</param>
internal readonly struct Range(Index start, Index end) : IEquatable<Range>
{
	/// <summary>Represent the inclusive start index of the Range.</summary>
	public Index Start { get; } = start;

	/// <summary>Represent the exclusive end index of the Range.</summary>
	public Index End { get; } = end;

	/// <summary>Create a Range object starting from first element to the end.</summary>
	public static Range All => new(Index.Start, Index.End);


	/// <summary>Indicates whether the current Range object is equal to another object of the same type.</summary>
	/// <param name="value">An object to compare with this object</param>
	public override bool Equals(object? value) => value is Range r && r.Start.Equals(Start) && r.End.Equals(End);

	/// <summary>Indicates whether the current Range object is equal to another Range object.</summary>
	/// <param name="other">An object to compare with this object</param>
	public bool Equals(Range other) => other.Start.Equals(Start) && other.End.Equals(End);

	/// <summary>Returns the hash code for this instance.</summary>
	public override int GetHashCode() => Start.GetHashCode() << 17 ^ End.GetHashCode();

	/// <summary>Converts the value of the current Range object to its equivalent string representation.</summary>
	public override string ToString() => $"{Start}..{End}";

	/// <summary>Create a Range object starting from start index to the end of the collection.</summary>
	public static Range StartAt(Index start) => new(start, Index.End);

	/// <summary>Create a Range object starting from first element in the collection to the end Index.</summary>
	public static Range EndAt(Index end) => new(Index.Start, end);

	/// <summary>Calculate the start offset and length of range object using a collection length.</summary>
	/// <param name="length">The length of the collection that the range will be used with. length has to be a positive value.</param>
	/// <remarks>
	/// For performance reason, we don't validate the input length parameter against negative values.
	/// It is expected Range will be used with collections which always have non negative length/count.
	/// We validate the range is inside the length scope though.
	/// </remarks>
	[MethodImpl(MethodImplOptions.AggressiveInlining)]
	public (int Count, int Length) GetOffsetAndLength(int length)
	{
		var startIndex = Start;
		var start = startIndex.IsFromEnd ? length - startIndex.Value : startIndex.Value;
		var endIndex = End;
		var end = endIndex.IsFromEnd ? length - endIndex.Value : endIndex.Value;

		if ((uint)end > (uint)length || (uint)start > (uint)end)
		{
			throw new ArgumentOutOfRangeException(nameof(length));
		}

		return (start, end - start);
	}
}

#endif